Posted: 01/06/09 02:54 PM
Dodge Ram Speed Sensing
Question: I have a 1994 Dodge diesel automatic I bought in 2004. Periodically, the OD Off light would come on by itself. Alarming, but easily returned to normal by pushing the switch--the light would go off. However, over the past month, the speedometer has been bouncing around erratically with no relation to speed. It's never steady, just goes all over the place. At times, this seems to affect the gear the truck is in, as I can hear it downshift and the revs go up to over 2000. The OD Off light stays on, and pushing the button off and on does nothing. Since I'm not independently wealthy, I don't have money to throw at someone who doesn't know what to do. I read in your column about a 1993 Dodge TPS (throttle position sensor) problem, which sounds similar. Do I need to go to an electrical specialist or the Dodge dealer, or should a good mechanic be able to look at this and address it in a cost-effective fashion?
Answer:I may be a bit biased--having worked the line in a N.Y. service department for more years than I care to mention--but here's my position: A situation like yours may be a quickie job for an experienced technician at Dodge service or a complex problem that'll require that same tech to make the repair efficiently. Keep in mind that these guys are specialists. Dodge products may be all they work on, they receive advanced education, and, typically, no one can fix a Ram better. Let's say the dealership service department charges a 50-percent-higher labor rate than a local independent shop. If the Dodge tech gets the job done in one labor-hour, versus two by a less knowledgeable outside tech, you're still way ahead of the game. Simple services, such as oil changes and rotating the tires, are another story altogether. There, you may certainly save money outside the dealership's service bays. The TPS can have an effect on transmission operation, but not the speedometer flipping out. Everything seems to point to an issue with the vehicle speed signal, which originates at the vehicle speed sensor mounted at the transmission. This speed signal is converted and used in different sub-systems such as electronic transmission and speedometer operation. The one thing that strikes me as odd is your speedo jumping to 60 while the vehicle's not moving. The loss of a speed signal will typically cause the needle to go down, not up. Jumping all over the place is a more common symptom of an instrument-cluster failure. Who knows, maybe there are two problems to deal with. The first thing to do is have a technician scan the system for trouble codes to help narrow down the cause, even if the problem is not occurring while in the shop.
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex Steele, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 6420 Wilshire Blvd., Los Angeles, CA 90048, or e-mail us at firstname.lastname@example.org.
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